the easiest thing to do, is start discussion for it or something else and upload file there... (just like the upload file button here, in the reply section)
other than that, I think you would have to upload file to some sort of server, i.e. myspace, photobucket, that type of stuff... but problem being - those places usually only allow image files, not excel... so you would have to find a place that lets you upload files. if you did find one of those, then simply post a link to where the file is you loaded on your mybag area
I would think, along those lines, change the excel file into an image file by either taking a screen shot of it or there is this web site www.zamzar.com that does file conversions and may be able to do it for you.